Stable undercooled metallic particles for engineering at ambient conditions

ABSTRACT

Undercooled liquid metallic core-shell particles, whose core is stable against solidification at ambient conditions, i.e. under near ambient temperature and pressure conditions, are used to join or repair metallic non-particulate components. The undercooled-shell particles in the form of nano-size or micro-size particles comprise an undercooled stable liquid metallic core encapsulated inside an outer shell, which can comprise an oxide or other stabilizer shell typically formed in-situ on the undercooled liquid metallic core. The shell is ruptured to release the liquid phase core material to join or repair a component(s).

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/299,682, filed Mar. 12, 2019, which is a divisional of U.S. patentapplication Ser. No. 14/999,868, filed Jul. 12, 2016, which claimsbenefit and priority of provisional application Ser. No. 62/231,722filed Jul. 14, 2015 the disclosures and drawings of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for joining or repairingcertain metallic components under ambient conditions using undercooledmetallic core-shell particles to form a metallurgical and/or mechanicalbond.

BACKGROUND OF THE INVENTION

Multi-dimensional fabrication by joining and/or material healing(repair) is limited, in part, due to inherent scale-based challenges inavailable tools. Most common defect repair or joining techniques likesoldering and welding, are limited to bulk uses and cannot be adopted tosmaller sizes especially at the microscale and smaller.

Ability to join, or fuse, materials is ubiquitous to manufacturing inmany fields such as; electronics, chemical, energy, medical, aerospace,defense, among others, but has been facing challenges, in part, due toincreased interest and advances in miniaturization, coupled with theneed for greener processes^([1,2]). Nanotechnology, for instance, hasgrown significantly in the recent past, and produces high performancematerials with many desirable properties. There are, however, obstaclesto fully actualizing the potential of nanomaterials because oflimitations in fabricating complex structures and devices^([3]).Recently, there has been efforts for interconnections of nanomaterialsby welding (e.g, cold welding^([4]), fusion welding^([5]), plasmonicwelding^([6])), soldering (e.g. liquid-phase reflow soldering^([7]),resistance soldering^([8]))), brazing^([9]) and others processes^([10])that heavily rely on in situ manipulation, directed assembly, and,self-assembly. The techniques, however, suffer from low efficiency, highcosts, and often need specialized samples (e.g. contamination-free, flatsurface, and, high purity depending on the technique), as such, they arefar from adoption in large scale manufacturing. Similarly, formicrosystems, joining is anticipated to be a significant hurdle in theiradoption in large-scale manufacturing and fabrication.^([1,11])

In addition to miniaturization issues, the joining industry has beenfacing two other challenges, viz; (1) Despite the well-establishedlead-bearing solders that have been used extensively in the assembly ofmodem electronic devices, limitations of lead use due to environmentaland health concerns has triggered research on alternative lead-freesolders. Lead-free solders, however, often require higher processingtemperatures than lead-containing solders (>450 K) which limits theiruse and increases cost. (2) Developing flexible electronics, polymerbased substrates, electronic devices, and, temperature sensitivecomponents (such as LEDs) require creating joints at low processingtemperatures^([12,13]). Also, demand for less energy consuming or moreenergy efficient processes has been increasing. Therefore, practicalenergy efficient joining and manufacturing techniques, with lowprocessing temperature, enabling fabrication of complex structure at themicro and smaller size scales is essential for future developments indevice manufacturing. It has previously been suggested that metalnanoparticles can be used as a solder material since the meltingtemperature decreases with reduction in particle size.^([13,14])

Undercooling of metals (i.e. cooling of a liquid metal or alloy belowits freezing point without it becoming solid, also known as“supercooling”) has been widely studied, primarily to inform metalprocessing and microstructure evolution during solidification.^([15,16])Due to the metastable nature of undercooled metals, their production ingood yields is an experimental challenge. This challenge can be overcomethrough; i) elimination of heterogeneous nucleating sites, or othersites with high catalytic potency for solidification, and, ii)minimizing the container effects by employing the droplet dispersion orcontainerless techniques in synthesis of undercooledparticles.^([17,18]) Using these techniques, undercooling values as highas about 0.3-0.4 T_(m) have been reported.^([16,19]) One of the highestundercooling achieved so far is 0.7 T_(m) for 3-15 nm galliumparticles.^([20]) The literature on undercooling, however, is heavilyskewed towards studies on understanding the solidification behavior andthermodynamics of metal systems.

There is limited discussion on practical applications except for heattransfer^([21]) and production of metastable solids.^([17]) One of thereasons for lack of practical use could be the challenges in preparingstable undercooled particles in high yields and at any size scaleespecially where large undercooling values are desired. In thecontainer-less drop tube technique, for example, the particle isundercooled during free fall. Droplet emulsion techniques, on the otherhand, allow for the production of more than one particle at once only ifthe carrier liquid can maintain a thin, inert surface coating inhibitcrystallization, however, stability is still a major concern.

A so-called Shearing Liquids Into Complex ParticlEs process (known asthe SLICE technique)^([13]) involves use of a rotating implement toshear a liquid metal, that is liquid at room temperature, into smallerpieces in an acid containing carrier fluid as illustrated schematicallyin FIG. 1 . Under SLICE, the liquid metal is sheared to the desired sizewith concomitant surface oxidation to give a passivating layer on whichan organic layer is assembled to give smooth surfaces, which is a keycomponent in efficient particle assembly.

The SLICE process is described in PCT/US14/69802 filed Dec. 11, 2014,and a related technique has been reported in U.S. Pat. No. 4,042,374issued Aug. 16, 1977

SUMMARY OF THE INVENTION

The present invention involves using undercooled metallic core-shellparticles whose undercooled core is stable against solidification atambient conditions, i.e. under ambient temperature and pressureconditions, in the joining or repairing of metallic non-particulatecomponents (i.e. components that do not have a particle shape). Theundercooled core-shell particles in the form of nano-size or micro-sizeparticles comprise a undercooled stable liquid metallic coreencapsulated inside an outer shell, which can comprise an oxide or otherstabilizer shell typically formed in-situ on the undercooled liquidmetallic core. The particles preferably can be stabilized in themeta-stable state using self-passivating oxide layer with a stabilizingorganic or inorganic adlayer to form a core-shell particle structure.

In an illustrative embodiment of the present invention for joining underambient conditions, the undercooled core-shell particles are assembledin a manner to join non-particulate metallic components and then theouter shells of the undercooled core-shell particles is ruptured torelease the undercooled liquid metallic material of the cores to contactthe components and solidify to produce a metallurgical joint between thecomponents. For purposes of further illustration and not limitation, thecomponent can include a metallic film or other material such as apolymer, ceramic, crystals, glass, inorganic material, or a compositematerial.

In another illustrative embodiment of the present invention forrepairing a defect of a non-particulate component, such as a surfacecrack, pit, depression, or other defect of a sheet or film, underambient conditions, the undercooled core-shell particles are assembledin a manner to fill the defect and then the outer shells of theundercooled core-shell particles is ruptured to release the undercooledliquid metallic material of the cores to fill the defect and solidifytherein.

In practicing these and other embodiment of the present invention,micro-machining, and/or fracturing through mechanical stressing, andselective chemical etching of the outer shells of the particlesinitiates a cascade of metallic liquid flow from the particle cores toprovide alloying and solidification. If the shell is ruptured usingmechanical stress, there is a concomitant deformation of the shellsproviding a combination/alloying, shaping, and, solidification.

This facile and low cost method pursuant to embodiments of the presentinvention, designated as SUPER (Stable Undercooled Particles forEngineering at Room temperature), enables joining or surface repair ofcomponents such as metallic films, wires, electrical connectionelements, that are not particles in shape at ambient conditions withoutheating, skilled manpower, high tech instrumentations, or, complicatedsample preparation procedures, or the need for surface cleaning (flux)reagents.

The present invention envisions a liquid metallic core-shell particlehaving an undercooled liquid metallic core comprising a metal or alloyhaving a melting point in the range of 62° C. to 900° C., such as 62° C.to 250° C., an outer stabilizer shell on the undercooled liquid metalliccore, and an inorganic or organic adlayer on the shell. The liquidmetallic core can comprise a solder alloy, such as a Bi-based solder, aSn-based solder, and other solder alloys having a melting point in therange of 62° C. to 250° C.

These and other advantages of the present invention will become morereadily apparent from the following detailed description taken with thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic of the SLICE process, where a rotating implementshears the liquid or molten metal into smaller pieces in an acidcontaining carrier fluid. FIG. 1B is a SEM micrograph of nanoparticlesderived from the SLICE process. FIG. 1C is an SEM image of anundercooled microparticle. FIG. 1D is a schematic of partialcross-section of a particle having an outer oxide or stabilizer shellsor layers encapsulating a liquid metallic core.

FIG. 2A is an SEM image of undercooled Field's metal microparticles.FIG. 2B are SFM images of two assembled undercooled metal particlescoalescing upon milling to form one large particle. FIG. 2C is an SEMimage of the uniform-composition particles after milling. FIG. 2D slowsthe contras between SEM images of an undercooled metal particle (bottomparticle) and a solidified one not pursuant to the invention (topparticle).

FIG. 3A is a schematic diagram of an illustrative embodiment of theinvention of method steps for repairing a defect of a metallic (e.g,silver) film or sheet wherein FIG. 3A illustrates experimental procedurefor healing a damaged silver surface. Undercooled particles were placedon a damaged area, sheared using glass cylinder, then template strippedto obtain a flat surface. FIG. 3B shows SEM image of a healed silversurface. FIG. 3C shows an elemental EDS map of Ag of the healed surface.

FIGS. 4A and 4B illustrate application of SUPER for joining where FIG.4A illustrates the experimental procedure for joining gold films byapplication of shear stress on undercooled particles sandwiched betweenaluminum foil leading to joining of the thin gold films with concomitantdelaminating from the aluminum foil. FIG. 4B are the low and highmagnification images showing the Field's metal particles joining thegold sheets while delaminating the aluminum foil.

FIG. 5 is an SEM of undercooled Bi—Sn particles pursuant to theinvention (lower particles) and phase segregated Bi—Sn eutecticsolidified particles P′ (topmost large particle) not pursuant to theinvention produced as described below.

FIG. 6 is a schematic diagram of use of the core-shell particles in ametal casting or molding process.

FIG. 7 is a schematic diagram of use of the core-shell particles in anadditive manufacturing process, such as 3D printing.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves a method using undercooled metalliccore-shell particles whose undercooled liquid core is stable againstsolidification at ambient conditions, i.e. under ambient temperature andpressure conditions, in the joining or repairing of metallic and/ornon-metallic material components such as films, wires, and particles ofregular or irregular shapes. The undercooled core-shell particles in theform of nano-size or micro-size particles comprise a stabilizedundercooled liquid metallic core encapsulated inside an outer shell,which can comprise of an oxide or other stabilizer shell(s) or layer(s)typically formed on the undercooled liquid metallic core. Undercooledparticles in the size range of 4 nm to 900 μm can be used in practice ofthe invention for purposes of illustration and not limitation. The outershell may be functionalized with an organic or inorganic moiety tofacilitate self-assembly as needed for a particular application.

The liquid metal or alloy of the core is encapsulated as an undercooled(supercooled) stable nano-particle and/or micro-particle, then used as alow temperature joining or repairing material, such as a solder, filler,etc. The use of undercooled metals significantly below their meltingpoint, T_(m), eliminates many drawbacks of currently available joiningor repairing techniques. The liquid metallic material of the undercooledparticle cores can have a melting point T_(m) in the range of 26 to 900°C. By using metals or alloys with low melting temperatures in theirundercooled state; e.g. a T_(m) less than 250 degrees C., then joiningor repairing can be performed in ambient temperature and pressure. Ajoint produced by practice of the method can include shell fragments ina metallic matrix comprising the metallic material of the cores andproducts of any stabilizing functionalizing organic or inorganic moietyor layer of the shells.

The undercooled metallic core-shell particles preferably are made by theaforementioned SLICE process, FIG. 1A, which is an extension of dropletemulsion technique (DET), although the present invention can bepracticed with liquid metallic nano-particles or micro-particles made byany other technique. The SLICE process involves shearing a low T_(m)molten metal or alloy in the presence of a carrier fluid as shown inFIG. 1A together with chemical reaction to produce liquid metalliccore-shell nano-particles or micro-particles P, FIGS. 1A, 1B, 1C, and1D. The chemical reaction typically involves oxidation of the moltenmetal or alloy in a manner to form the outer shell in-situ on the liquidmetallic core. Moreover, the outer shell can be functionalized with anorganic moiety, such as acetate or phosphate, to facilitateself-assembly as needed for a particular application, FIG. 1D. Thepresent invention thus envisions a method to prepare stabilizedundercooled particles in high yields by shearing liquids metals into adesired sizes and stabilizing the meta-stable state usingself-passivating oxide layer with a stabilizing organic or inorganicadlayer to form a core-shell particle structure.

For purposes of illustration and not limitation, subjecting Field'smetal melt (or other metal or alloy) to SLICE in the presence of thedilute (about 2%) acetic acid in diethylene glycol as described belowleads to production of copious amounts of particles, bearing differentsurface architectures wherein a majority of the particles have a smoothsurface texture, FIG. 2A, evidencing production of desired undercooledliquid metallic core-shell particles with a few particles showingphase-segregation. FIG. 2A shows two such desired undercooled liquidmetallic core-shell particles with homogenous chemistry. FIG. 2B showsSEM's of the particles in stages during milling. FIG. 2C shows theparticles after milling. Associated EDS elemental maps (not shown) ofBi, In, and Sn of the particles demonstrate that the particle ishomogenous in elemental composition. FIG. 2D shows the contrast betweenSEM images of an undercooled metal particle pursuant to the invention(bottom particle) and a solidified one (top particle). Elemental EDSmaps (not shown) demonstrated that there is phase-segregation uponsolidification of the eutectic particle (top particle), but no phasesegregation occurs for the undercooled particle (bottom particle), alsodemonstrating homogeneously distributed constituent elements (i.e.bismuth, indium and tin) and indicating that the encapsulatedundercooled material particle is composed of one phase. An inherentadvantage of SLICE is that the size and polydispersity of the generatedspherical particles can be controlled through felicitous choice of thesynthesis conditions.

Field's metal solidifies into different combination of phases (possiblephases include: β, γ, BiIn₂ and/or quenched liquid) depending on thesolidification conditions^([24],[25]). Thermodynamics andmicrostructures of the eutectic Bi—In—Sn alloys have been discussed indetail by Witusiewicz et al.^([25]) and Çadirh et al.^([24]), which areincorporated herein by reference to this end. The surface texture andphase segregation phenomena of Field's metal can be used to distinguishsolidified particles from the desired undercooled liquid metalliccore-shell particles. It has previously been shown that undesiredsolidification of the melt leads to surface phase segregation andsubsequent roughening of the metal surface.^([22]) Undesirablesolidified particles can be identified and confirmed by EDS bycharacteristic contrast in grey scale due to phase-segregation and bytheir rough surface due to volume changes during solidification. Incontrast, undercooled are characterized by a smooth uniform surfacetexture and homogeneous distribution of elemental constituentsparticles.

Removal of the outer layers of the nanoparticles by for example millingwith FIB (focused ion beam) leads to flow and coalescence of theundercooled liquid cores of the particles, demonstrating that thecore-content is a liquid. Having this liquid phase of Field's metal(T_(m)=62° C.) at room temperature shows that the metal is undercooled.Homogeneous distribution of constituent elements in the alloy, evenafter coalescence under high vacuum indicates that solidification isprevented probably due to lack ofa nucleation points or solidificationcatalysts to help generate nucleation sites on the surface, although theinventors do wish to be bound by any theory in this regard.

The desired undercooled liquid metallic core-shell particles can besuspended in a fluid, such as ethanol, and moved without specificprecaution and stored for days at a time as evidenced by the samplepreparation steps (centrifugation, filtering, vortex mixing) prior toSEM imaging described below in connection with the Examples. Thus, itcan be inferred that the desired undercooled liquid metallic core-shellparticles are fairly stable and amenable to workflow and movingstresses. Even direct contact with a rough surface does not inducecrystallization, mainly because the outer organic layer (attached on theoxide, e.g. FIG. 1D) isolates the undercooled liquid from contact withthe rough surfaces that are potential nucleation points. Stability ofthese undercooled particles gives an opportunity to manipulate.Moreover, since the solidification is possible at room temperature oncethe outer layer is removed, these particles can be used for joining andrepairing of nano- and micron-size systems at ambient conditions asdemonstrated in the Examples below.

In practice of embodiments of the present invention, the undercooled,liquid metallic core-shell particles are used as a joining platform orrepairing platform at ambient conditions wherein the outer shells areruptured by gentle micro-machining (e.g. focused ion milling),fracturing through mechanical stressing, and/or selective chemicaletching to initiate a cascade entailing liquid metal flow from the coreswith concomitant deformation, combination/alloying, shaping, and,solidification. If the shell is ruptured using mechanical stress, thereis a concomitant deformation of the shells providing acombination/alloying, shaping, and, solidification.

Illustrative examples of shell rupture include, but are not limited to,(i) fracturing of the outer shells by focused ion beam (FIB) milling;(ii), mechanically stressing the particles resulting in fracture ofouter shells; and (iii) chemical etching a region of the outer particleshells using an appropriate acid. Rupture of the outer shells or layersleads to flow and subsequent coalescence of the undercooled liquid metalor alloy of the core. Instantaneous solidification occurs due to thenumerous in situ generated nucleation sites, in part, due to; i) oxidefragments, ii) contact with substrate surface walls, iii) rapidoxidation in air, and, iv) equilibration to ambient conditions.

In an illustrative embodiment of the present invention for joining inair under ambient conditions using SUPER, the method involves assemblingthe undercooled core-shell particles in a manner to join non-particulatemetallic components and then rupturing the outer shells of theundercooled-shell particles to release the undercooled liquid metallicmaterial of the cores to contact the components and solidify to producea mechanical and/or metallurgical joint between the components. Forpurposes of further illustration and not limitation, referring to FIG.4A, the undercooled particles are assembled as a particle layer betweenfirst and second thin sheets of a metal or alloy (e.g. Au backed by Alin the figure) to form a sandwich-type preform and then a normal stress(stress applied orthogonal to the plane of the sheets) is applied to thesandwich-type preform to fracture the outer particle shells to releasethe liquid metallic core material for solidification between the sheetsto join them together. The metal or alloy comprising the liquid metalliccore typically alloys or amalgams with the material comprising thesheets to be joined to form a metallurgical bond. Mechanical bonding,upon breakage of the protective shell and solidification, can also beformed to facilitate joining where chemical bonds are not possible.Joining of such sheet-like laminates by rupture of the particle shellfollowed by coalescence and solidification of the released undercooledliquid from the particle cores is referred herein to as “lava flowsoldering” (LFS). When the particles are subjected to mechanical stressto rupture the particles, a joint produced by LFS.

The thin metal sheets or films comprise a metal or alloy that may or maynot be chemically different from that of the metal or alloy of theparticle cores. For example, gold thin films can be joined by Field'smetal released, coalesced and solidified therebetween as described inthe Examples below. Sheets having a thickness of 200 nm can be joined inthis manner for purposes of illustration and not limitation.

In another illustrative embodiment of the present invention forrepairing a defect of a non-particulate component that penetrates to thecomponent surface (e.g. a crack, pit, etc.) using SUPER, the undercooledcore-shell particles “Particles” are assembled in a manner to at leastpartially fill the defect designated “Damage” as illustratedschematically in FIG. 3A where the defect is shown in a Ag film or layeron a Si substrate. Then, the outer shells of the undercooled particlesare ruptured by shearing, for example, by rolling the glass pipette rodshown over the particles, to release the undercooled liquid metallicmaterial of the cores to fill the defect and solidify therein, all inair under ambient conditions of temperature and pressure. When theparticles are subjected to mechanical stress to rupture the particles, arepaired region is produced by LFS. A glass piece can be placed over therepair to separate the repaired Ag film by template stripping asdescribed in the REPAIRING example below.

In the above illustrative embodiments, the undercooled core-shellparticles can be used to trap or agglomerate particulates larger orsmaller in size than they are.

The following Examples are offered to further illustrate practice of thepresent invention, but not limit the scope of the invention.

EXAMPLES

Materials Used:

Eutectic compositions of bismuth-indium-tin (Field's metal, Bi:In:Sn32.5:51.0:16.5 wt %, m.p.≈62° C., Alfa Aesar, where its freezing pointis generally equal to its m.p.) and bismuth-tin (Bi:Sn 58:42 wt %,m.p.≈139° C., Alfa Aesar) were used. For particle preparation, aceticacid (Biotech, sequencing grade), diethylene glycol (BioUltra) andethanol (200 proof) were purchased from Fisher, Sigma, and DeconLaboratories Inc., respectively.Particle preparation: The SLICE method was followed to form particleswith liquid metallic core and oxide-acetate outer layer. An amount [0.6g (approx.)] of the liquid metallic material was added in acetic acidsolution (2 vol % for Field's metal, 1 vol % for Bi—Sn particles) indiethylene glycol. The solution prepared in a glass vial (scintillationvials, 20 mL) were kept in oil bath at determined temperature (120° C.for Field's metal and 160° C. for Bi—Sn) for at least 2 min beforesubjected to shear to ensure metal melt. Shear was applied using aDremel 3000 variable speed rotary tool at the rate of 17,000 rpm withextender accessory and cross-shaped (or any other desired geometry)poly(tetrafluoroethylene) (PTFE) shearing implement. Shearing implementwas placed as close as possible to vial wall to enhance the effect ofshear. After 10 minutes of continuous shearing, heat was withdrawn, andthe shear rate was gradually decreased to zero over a period of 1minute. The suspension was allowed to gradually cool to about 10° C.above the metal's melting point before filtration and washing. Excessacetic acid and diethylene glycol was washed out with ethanol throughfiltering. Whatman #1 (particle retention of 1 μm), VWR Filter paper 494(particle retention of 1 μm) and Whatman grade EPM 2000 (particleretention of 0.3 μm) filter papers were used for separation and cleaningof particles. Particles were stored in ethanol.Focused ion Beam: Scanning electron microscopy-Focused Ion Beam(SEM-FIB): Zeiss NVision 40 Dual-Beam SEM-FIB was used to image theformed particles and mill away their surfaces. Imaging was performed at2 kV with a working distance of 5.1 mm tilted at a 54° angle with apixel size of 6.602 nm, Images were collected using the In-lensdetector, A FIB of gallium ions was used to mill away a rectangular areadirectly over the formed EGaIn particles using an accelerating voltageof 30 kV and ion current of 1 pA. Milling was performed one frame at atime followed by imaging with the SEM.Particle Characterization:

In the above Examples, all metal articles were characterized withscanning electron microscopy (FEI Quanta 250 FE-SEM). The SEM wereoperated under high vacuum at the voltage of 8-10 kV. Both the secondaryelectron and the energy selective backscattering (EsB) mode were used toimage the samples. Chemical characterization were conducted by energydispersive X-Ray spectroscopy (EDS). Additional characterization wasperformed on a Zeiss Supra 55VP Field Emission SEM. Samples were imagedusing an electron beam accelerating voltage of 3 kV and a workingdistance of 3.3 mm. Images were collected using an In-lens detector oran Everhart-Thornley secondary electron detector. Elemental analysis wasperformed at a working distance of 8.5 mm and using electron beamaccelerating voltages of 15 kV. Elemental composition was determinedusing an Energy Dispersive x-ray Spectrometer with a silicon driftdetector.

Repairing and Joining:

In some examples, these undercooled core-shell particles were used forhealing of damaged surface, such as a crack, scratch, or other defectbelow the microscale when the surface bearing the defect or to be joinedcan alloy or amalgam with the undercooled metal. This approach can bevaluable in repairing delicate thin film materials where hightemperature or mechanical force cannot be applied. In other examples,the undercooled core-shell particles were used for joining metals oralloys having chemistry(s) different from the chemistry of theundercooled particles. Selective joining can be realized where the metalor alloy can alloy or amalgam with particle alloy or even non-alloyingmaterials that form weak mechanical contacts

Repairing:

A 200 nm thick pure silver (99.99%) films were deposited on siliconwafer using e-beam evaporator (Temescal BJD-1800). Silver layer weredamaged to create a ˜millimeter-wide defect by cutting off a section ofthe metallic film. A suspension of SLICE-produced undercooled core-shellparticles in ethanol were poured on silver surface to form a mass ofparticles filling the defect, FIG. 3A, the solvent was allowed toevaporate at ambient conditions, and application of mechanically shearstressed using side of a glass Pasteur pipette after evaporation ofethanol to released the liquid phase core alloy, which alloyed with theAg film to seal the defect, FIG. 3B. Then, template stripping procedurewas applied to remove the healed silver film from the silicon substrate,FIG. 3A. For template stripping, a glass piece was cleaned with ethanoland dried with a stream of nitrogen gas. An estimated 5 μL of opticaladhesive (Norland optical adhesive 61) was applied on a glass piece andglued on a substrate. The sample was exposed to UV light to cure anadhesive for 12 hours. The glass stripped off the substrate using arazor blade.

Elemental EDS maps of Ag, Bi, In and Sn showed that the Field's metalalmost fully recovered the damaged area of the silver sheet FIG. 3Cshows EsB detector image of a healed silver surface and shows elementalEDS map of Ag. This type of joining is designated as ‘lava-flowsoldering (LFS) [also referred to as “lava-flow welding” (LFW)] since itentails flow and solidification of the undercooled metal. As clearlyseen in FIG. 3B, the damaged area is healed with the solidified metalupon mechanical stressing the collection of undercooled particlesfilling the defect. Compositional distribution of Field's metalconstituents, Bi, In, and Sn, was not homogeneous over the area covered,which probably stemmed from the separation of components due to kinetic-and/or thermal-differentiation of the resultant alloy with Ag. Moreuniform healing could be obtained by separation of bismuth-richundercooled particles owing to Bi having relatively higher density. Inthis example, the healing materials were selected to be different inchemistry than the surface to increase the contrast between phases, butbetter alloying elements can be achieved using undercooled particlesfrom the same metal.

Lava Flow Soldering:

Another example of the application of SUPER is joining of thin Au films(200 nm thick) with concomitant delamination from the substrate (FIGS.4A and 4B). For this experiment, aluminum foil (Reynolds Wrap) wasdeposited with a 200 nm thick gold (99.99%) film using the same e-beamevaporator system described above. Undercooled particles were sandwichedin between the foil prior to folding to fold the gold film onto itself.The undercooled core-shell particles in ethanol solution were dropped onthe gold foil using a Pasteur pipette. The folded aluminum foil/goldfoil was sheared (mechanically stressed) by side of a glass Pasteurpipette to join the two gold surfaces.

As a result, the released liquid Field's metal not only alloyed with andmetallurgically joined together the facing Au sheets, FIG. 4B, but alsodelaminated the joined gold films from the aluminum foil support,indicating a stronger (thermodynamically favorable) interaction withField's metal. Elemental EDS maps across the joint that show that thedistribution of Al, Au, Bi, In, and Sn was in accordance with FIG. 4B.

This example demonstrates that layered lamina material can be step-wisedelaminated using this so-called lava flow soldering where undercooledcore-shell particles specific to each layer are used. Similarly, usingless and/or smaller particles and tuning the applied stress as discussedabove, many different joints can be obtained at ambient conditions. Ahigh normal (orthogonal) stress in this example was used to allow forimaging, but rapid application of high shear stress gives the same LFWeffect but with a thinner inter-layer between the gold films.

The present invention envisions a liquid metallic core-shell particlefor use in soldering applications and having an undercooled liquidmetallic core comprising a metal or alloy having a melting point in therange of 62° C. to 900° C., such as 62° C. to 250° C., an outerstabilizer shell on the undercooled liquid metallic core, and aninorganic or organic adlayer on the shell. The liquid metallic core cancomprise a solder alloy, such as a Bi-based solder such as Field'smetal, a Sn-based solder, and other solder alloys having a melting pointin the range of 62° C. to 250° C.

Another Example of practice of the present invention involves a methodinvolving embedding or dispersing the above-described undercooledparticles in any material such that, upon failure, the particles ruptureand the liquid metallic core material flows and solidifies to stoppropagation of the defect (e.g. crack) caused by the failure withsubsequent healing of the already-formed defect of the material.

Still another Example of practice of the present invention involves amethod of preparing composite materials in which the undercooled liquidmetallic core-shell particles are used to introduce a metal into acomposite material. The particles are ruptured in the material to createand exploit mechanical and/or chemical bonds with the material toproduce a composite material. The undercooled core-shell particles canintroduce the matrix of the composite material or supply the filler (orco-filler) material. The core-shell particles can be used to createself-repairing composite materials, in which case the particles ruptureupon mechanical failure of the composite and upon flow andsolidification of the core material stop propagation of a defect (e.g.crack) caused by the failure and heal the defect.

A further embodiment involves using the undercooled particles to trap oragglomerate particulates larger or smaller in size than they are bymixing the undercooled particles and the particulates and then rupturingthe shells so that the liquid metallic core is released and traps oragglomerates the particulates as a mass upon solidification.

Still other applications of SUPER are set forth below

Metal Casting or Molding

Another example of the application of SUPER is metal casting or moldingwherein the above-described core-shell particles P are placed in a mold100, FIG. 6 . In metal casting, pressure is applied to the particles Pin the mold to rupture the outer shell and release the undercooledmolten metallic core material to solidify in the mold as a cast bodyshaped by the inner shape of the mold. Pressure can be appliedmechanically to the particles using a piston or plunger 102 moved intothe mold 100 to press on the particles or non-mechanically usingvibration, ultrasound or other pressure applying technique to rupturethe particle shells in the mold.

In metal molding, the above-described core-shell particles are placed ina female part of a mold apparatus, such as mold 100 of FIG. 6 . In metalmolding, pressure is applied to the particles in the female mold byinserting and moving the male part, such as mold piston or plunger 102of the molding apparatus, FIG. 6 , to rupture the outer shell andrelease the molten metallic core material to solidify as a molded body.

Additive Manufacturing (e.g. 3D Printing)

Still another example of the application of SUPER is 3D printing whereinthe above-described core-shell particles are disposed in a carrier fluidfor discharge from a nozzle 300, FIG. 7 . The discharged core-shellparticles in the carrier fluid can be immediately stressed as they leavethe nozzle 300 in a manner to rupture the particle shell so that theliquid metallic core material is deposited on a substrate. Stress can beapplied on the discharged core-shell particles by a pressure change upondischarge from the nozzle or by a laser beam(s) from laser(s) 302directed at the discharged particles to cleave the outer shell in amanner used in practice of the known laser sintering process or inpractice of the so-called laser induced forward transfer (LIFT) processfor laser jetting of metal droplets (see Bohandy et al., “Metaldeposition from supported metal film using excimer laser”, J. Appl.Phys., 60, 1538 (1986) and Mogyorosi et al., Pulsed laser ablativedeposition of thin metal films: App. Surf. Sci. 36, 157-163 (1989), thedisclosures of which are incorporated herein to this end.

Alternately, the core-shell particles can be discharged from the nozzle300 without rupturing the outer shells so that the core-shell particlesare deposited on substrate where the particle shells are ruptured uponimpact with the substrate surface or by subjecting the core-shellparticles after deposition to a mechanical or non-mechanical rupturingstep to rupture the outer shell and release the molten metal corematerial to solidify in-situ on the substrate as a printed layer. Forexample, the outer shells of as-deposited particles can be ruptured byapplying a mechanical stress in the thickness direction to the particleson the substrate using suitable pressure-applying means described aboveor by non-mechanical laser cleaving or ablation of the outer shells.

The above Examples illustrate use of the SUPER technique in whichundercooled core-shell particles are used for rapid, heat-free joining,repairing and structure fabrication (casting/3D printing) at ambientconditions such as room temperature. The capability of this techniquewas demonstrated with respect to defect healing and lava flow soldering.Applications of SUPER, however, are not limited to these specificexamples. For example, use of eutectic bismuth-tin alloy particlesproduced by SLICE can be used as candidate lead-free solders^([27]) foruse in soldering electrical connections, such as wire-to-wire,wire-to-contact pad, and other complex geometries. Even though BiSn ateutectic composition has much higher melting point (m.p. 139° C.) thanField's metal (m.p.≈62° C.), undercooled particles can be produced, FIG.5 , at high yields, using SLICE as described above in the “Particlepreparation” paragraph and employed as a Bi-based solder in solderingsuch electrical connections in a manner described above for LFW joining.The undercooled particles can be assembled as a mass or mound or as alayer over the electrical connection where the components are to bejoined (soldered) and then ruptured there to form the solder joint asdescribed above.

In practicing the above described Examples, it has been found thatsuperheating the alloy during the SLICE process to at least 20 degreesC. above the melting point T_(m) of the metal or alloy producesbeneficial results in terms of suitable undercooled liquid core-shellparticles that are stable enough at ambient or room temperature toenable heat-free soldering and repairing as described above. Metals andalloys with higher T_(m) can be made using embodiments of the inventionwherein the SLICE liquid comprises a material such as an ionic liquid[e.g. (BMIM)(PFNFSI) which decomposes at T_(d) of 290° C. and quaternaryammonium ionic liquids]; or polar hydrocarbon liquid (e.g. polyphenylether pump fluid, boiling point T_(b) approximately 475° C. at 760 mmHg)having a higher melting point that allows the SLICE process to bepracticed at the suitable temperature that is 20° C. above thecorresponding Tm of the metal or alloy. (BMIM)(PFNFSI) is1-butyl-3-methylimidazolium N-pentafluorphenyinonatluorbutylsulfonamide.

Other ionic liquids having appropriate T_(m)'s that can be used belowtheir decomposition temperature (T_(d)) and above their T_(m) aredescribed in Zhang, Physical Properties of Ionic Liquids: Database andEvaluation, J, Phys. Chem. Ref. Data, Vol. 35, No. 4, 2006, thedisclosure of which is incorporated herein by reference, and include butare limited to, [NH₄][NO₃]; [TMA][BF₄]; [TEA][BF₄]; [TPA][BF₄];[TBA][BF₄] where TMA is tetramethyl ammonium; TEA is tetraethylammonium; TPA is tetramyl ammonium; and TBA is tetrabutyl ammonium. Suchalloys having a higher T_(m)'s can include, but are not limited to,solder alloys Sn₉₁Zn₉ (T_(m)=199° C.); Sn_(96.5)Ag_(3.5) (T_(m)=221°C.); and so-called SAC Sn_(96.5)Ag_(3.5-4.7)Cu_(0.5-1.7) (T_(m)=217° C.)and other Sn-based solders as well as a Ag-based or Ag-containing alloysolder and a Au-containing alloy solder.

Although the present invention has been described with respect tocertain illustrative embodiments, those skilled in the art willappreciate is not limited to these embodiments and that changes andmodifications can be made therein within the scope of the invention asset forth in the appended claims.

References, which are incorporated herein by reference:

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We claim:
 1. A liquid metallic core-shell particle comprising: anundercooled liquid metallic core comprising a metal or alloy, theundercooled liquid metallic core having a melting point in a range of62° C. to 250° C.; and an outer shell on the undercooled liquid metalliccore, the outer shell comprising an inorganic or organic adlayer,wherein the inorganic or organic adlayer comprises acetate, phosphate,or a combination thereof; wherein the undercooled liquid metallic coreis a liquid and the liquid metallic core-shell particle has atemperature that is less than the melting point of the metallic core. 2.The particle of claim 1, wherein the liquid metallic core-shell particlehas a temperature that is greater than room temperature and less thanthe melting point of the metallic core.
 3. The particle of claim 1,wherein the range of the melting point of the metallic core is in 62° C.to 221° C.
 4. The particle of claim 1, wherein the liquid metallic coreis a solder alloy.
 5. The particle of claim 1, wherein the liquidmetallic core comprises a solder alloy.
 6. The particle of claim 1,wherein the alloy is selected from the group consisting of an alloycomprising In, an alloy comprising Ga, an alloy comprising Pb, an alloycomprising Bi, an alloy comprising Sn, an alloy comprising Ag, and analloy comprising Au.
 7. The particle of claim 1, wherein the metal oralloy comprises at least one of gold and silver.
 8. The particle ofclaim 1, wherein the undercooled liquid metallic core comprises aBi-based solder.
 9. The particle of claim 1, wherein the undercooledliquid metallic core comprises a Sn-based solder.
 10. The particle ofclaim 1, wherein the undercooled liquid metallic core comprises Field'smetal, wherein the Field's metal is about 32.5 wt % Bi, about 51.0 wt %In, and about 16.5 wt % Sn.
 11. The particle of claim 1, wherein theundercooled liquid metallic core is stabilized against solidification inits undercooled state by the inorganic or organic adlayer.
 12. Theparticle of claim 1, wherein the inorganic or organic adlayer comprisesacetate.
 13. The particle of claim 1, wherein the outer shell comprisesan oxide.
 14. The particle of claim 1, wherein the outer shell comprisesa stabilizer shell, wherein the metallic core is on an on inside surfaceof the stabilizer shell, wherein the inorganic or organic adlayer is onan outside surface of the stabilizer shell.
 15. The particle of claim14, wherein the stabilizer shell comprises a self-passivating oxidelayer.
 16. The particle of claim 1, wherein the outer shell of theparticle is rupturable by machining, by applying a mechanical stress, orby chemical etching using an etching reagent.
 17. The particle of claim1, wherein rupturing the outer shell of the particle releases theundercooled liquid metallic core and solidifies the released undercooledliquid metallic core.
 18. The particle of claim 1, wherein the particleis located: between components to be joined upon breakage of the outershell of the particle, the components independently comprising ametallic material, a non-metallic material, or a combination thereof, ina surface defect for repair of the surface defect upon breakage of theouter shell of the particle, in a material to be formed into a compositematerial upon breakage of the outer shell of the particle, or in aself-repairing material for repair of the self-repairing material uponmechanical failure of the self-repairing material to break the outershell of the particle.